Ch 24 - Directional And Lateral Stability Flashcards
Directional Stability
Concerned with yaw about the normal axis
Lateral Stability
Concerned with roll about the longitudinal axis
Yaw and Roll Couple
Yaw can induce roll and roll can induce yaw
Static Directional Stability (N)
Is when the AC displays an initial tendency to return to straight flight after a disturbance in yaw. (Weathercocking - aligning itself to the airflow)
Mustn’t be too strong because the AC must allow the pilot to change flight direction easily
Sideslip Angle
The sideslip angle (yaw) angle is between the longitudinal axis and the RAF
Airflow coming from the right is a positive sideslip angle
Positive Directional Stability = yawning moment right
Yaw Moment Coefficient (Cn)
Fin has the greatest influence.
Cn = N (yawning moment) / q(dynamic p) x S(surface area) x b(wingspan)
Positive Directional Stability = AC will yaw towards the RAF
Neutral = no Cn whether sideslip angle increases/decreases
Negative = yaw away from RAF
EASA - at small and moderate sideslip angles, the AC must have static directional stability, at larger sideslip angles, will usually turn neautral or even negative
Factors affecting Directional Stability: The Fin
The biggest effect on Directional Stability. The larger the distance between the CG and the fin, along with how big it is, will produce a yawning moment (restorative) that yaws the AC towards the RAF
Restoring moment larger when: CG further forwards Fin surface larger High IAS Larger Sideslip angle
Fin Aspect Ratio and Sweepback
Straight fin with higher aspect ratio generates more side force for given AoA but has a lower stalling angle.
A swept fin or low aspect ratio fin produces less force for a given AoA but has a great stalling AoA
To lower aspect ratio, add a dorsal fin - increases stalling angle and improves Directional Stability.
Usually swept to the same angle as the wing for high speed flight reasons.
Ventral fins increase Directional stability at higher angles of attack (shielded)
Strakes
Long fins with relatively little vertical extent which are attached to lower sides of fuselage to improve directional stability at slow speed
Factors affecting Directional Stability: The Wing
Swept wing has a weak stabilising effect.
The into wind wing has a greater effective span which produces more lift which means more induced drag which yaws the AC to the RAF
Directional stability increases with Sweepback Angle
Factors affecting Directional Stability: The Fuselage
Has a destabilising effect overall.
Forward of the CG creates a larger destabilising effect
Aft of the CG creates a stabilising (restorative) moment
Static Lateral Stability
Positive static lateral stability - initial tendency to return to wings level in a roll
An aircraft must be signed to have positive static lateral stability
Bank Angle
Angle between the lateral axis and the Horizontal
The greater the bank angle in straight and level flight the larger the sideslip angle
The Roll Moment Coefficient (Cl’)
Cl’ = L(rolling moment)/ q(dynamic p) x S(surface area) x b(wingspan)
Positive roll moments describe a roll to the right when viewed from behind
Sideslip angle and Roll moment relationship
Positive Static Lateral Stability will roll in the opposite direction from the sideslip. Sideslip angle +, Rolling moment will be - (Stable)
Lateral stability is shown by a line with a negative gradient
Negative lateral static stability - roll the same direction as the side slip (+,+/-,-)
Neutral static lateral stability will generate no Cl’ whether sideslip angle increases or decreases,
Factors Affecting Static Lateral Stability: Dihedral Effect
The lower wing has greater AoA and thus produces more lift which rolls the aircraft back towards equilibrium.
Positive Static Lateral Stability (Acheived by; dihedral, sweepback, high mounted wing, high keel surface)
Anhedral produces the opposite effect which reduces lateral stability
Factors Affecting Static Lateral Stability: Sweepback
The lower wing has greater effect span than the upper wing so it generates more lift than the upper wing which tends to roll the AC back towards the equilibrium.
Factors Affecting Static Lateral Stability: High Wing
Aerodynamic centre on the high wing is vertically displaced from the CG so when ever the aircraft is banked, a restoring couple forms between the CG and the AC
Shielded upper wing means that the lower wing has increased upwash and thus the angle of attack of the low wing is increased which rolls the wing back towards equilibrium (restoring)
Factors Affecting Static Lateral Stability: High Keel Surface
The amount of fuselage side above the CG which is presented to the airflow.
The sideways component of RAF impacts the side of the fuselage and forms a couple with the CG which creates a restorative rolling moment.
Dihedral effect becomes stronger with increases sideslip
Spiral Instability
When directional stability is strong and the lateral stability is weak.
Wing drops, aircraft sideslips towards RAF, higher wing travels faster and therefore produces more lift, if lateral stability weak, AC then rolls towards lower wing which causes more sideslip, more yaw - all leads into a spiral dive with roll re-in forcing yaw and vice versa
Dutch Roll
Occurs when the lateral stability is strong and the directional stability is weak.
Initial disturbance sets up a gently oscillation patter of roll, yaw and sideslip.
If an AC suffers from weak directional stability and strong lateral stability, a yaw damper must be fitted. You can not take off if it is broken and must land ASAP if it breaks in flight
Effects of Pressure Altitude on Dynamic Stability
Dynamic stability reduces with an increase in pressure Altitude because the increases TAS for the same IAS results in a smaller sideslip angle.
At lower altitudes, the aerodynamic damping is greater that means the damping effects oppose control inputs more.
Roll/Yaw or corrective inputs are also less effective lower down
